High capacity finned tube heat exchanger

ABSTRACT

The fins of a finned tube heat exchanger extend in spaced parallel fashion across a plurality of fluid carrying tubes. The fins are bent into shallow angulated convolutions in the direction of flow of an external fluid passing between the fins and around the tubes. The fins carry a plurality of aligned openings within each convolution wall to increase heat transfer capacity with little increase in the fluid pressure drop of the external fluid flowing through the exchanger.

United States Patent 1191 Malhotra et a1.

[ Mar. 12, 1974 1 1 HIGH CAPACITY FINNED TUBE HEAT EXCHANGER 175]Inventors: Ravi K. Malhotra; Anthony M.

Castello, both of Harrisonburg, Va.

[73] Assignee: Dunham-Bush, lnc., Harrisonburg,

[22] Filed: Oct. 2, 1972 [21] Appl. No.: 293,892

[52] US. Cl 165/151, 29/151, 29/152, 1 1 29/153 [51] Int. Cl. F28d 7/00[58] Field of Search 165/151-153 [56] 1 References Cited UNITED STATESPATENTS 1,416,570 5 1922 Modine 165/151 1 1,739,672 12/1929 Higgins165/151 FOREIGN PATENTS OR APPLICATIONS 921,165 3/1963 Great Britain165/151 Primary EXaminerCharles J. Myhre Assistant ExaminerTheophil W.Streule, Jr. Attorney, Agent, or Firm-Sughrue, Rothwell, Mion, Zinn &Macpeak [5 7 ABSTRACT The fins of a finned tube heat exchanger extend inspaced parallel fashion across a plurality of fluid carrying tubes. Thefins are bent into shallow angulated convolutions in the direction of-flow of an external fluid passing between the fins and around thetubes. The fins carry a plurality of aligned openings within eachconvolution wall to increase heat transfer capacity with little increasein the fluid pressure drop of the external fluid flowing through theexchanger.

2 Claims, 3 Drawing Figures HIGH CAPACITY FINNED TUBE I-[EAT EXCHANGER'BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to finned tube heat exchangers, and more particularly to a finconstruction and configuration for increasing the heat transferefficiency of the heat exchanger.

2. Description of the Prior Art Finned tube heat exchangers consistessentially of a plurality of closely spaced sheet metal-finswith aplurality of refrigerant carrying tubes passing through the finsgenerally at right angles thereto. A high degree of heat transfer occursbetween the exterior fluid such as air which generally is in forced flowacross the surface of the fins and at right angles to the axis of thetubes and the fluid within the tubes by way of the fins. Such heatexchangers employ various tube arrangements and fin shapes in an attemptto achieve maximum heat transfer. Basically, the fin surfaces aredesigned to break the boundary layer formed on the surface of the finand to create turbulence in the air and thus increase heat transferbetween the refrigerant within the tubes and the exterior fluidpassingover the fin surfaces. Unfortunately, the heat transfer capacityof the heat exchanger increases atthe expense in air pressure drop tothe heat exchanger. One attempt at providing an increased heat transfercapacity by modification of the fin structure is set forth in US. Pat.No. 3,631,922, wherein openings are provided within the fin by strikinga plurality of tangs outwardly from the fins from one side thereof andtowards the adjacent fin, the tangs projecting through the air flow andcreating a desired turbulence to air flow through the heat exchanger andat right angles to the multiple refrigerant carrying tubes. The presenceof the tangs constitutes a lint trap and accelerates clogging of thecoil. Further, the presence of the tangs actually inhibit passage of theair through the holes resulting from the struck away tangs and theirpresence, while substantially increasing the turbulence of the airstream passing therethrough for increased heat transfer capacity,materially increase the pressure drop of the air stream across the heatexchanger.

SUMMARY OF THE INVENTION The improved fin structure and configuration ofthe fin and tube heat exchanger of the present invention has for itsobject a provision of a fin surface which has higher outside filmcoefficient with little increase in air pressure drop through the heatexchanger by diffusing and mixing the air as it flows through the heatexchanger. The fin is constructed such that the air flowing through theexchanger changes direction to remove more heat from the fin surface.The length of the flow path for the air flowing through the heatexchanger is increased to increase heat transfer between the fluids. Itpermits higher face velocities without condensate blow off problemswhere the heat exchanger is used within a dehumidifying application,while further providing a fin which is lighter in weight which acts toreduce the weight of the heat exchanger as a whole.

These objects are met in a preferred embodiment of a fin and tube heatexchanger in which a plurality of fluid carrying tubes pass throughaligned holes within a plurality of closely spaced, generally parallelfins formed of conventional thin metal sheet stock. Specifically, thefins are bent into convoluted form in the flow direction of the externalfluid and carry a plurality of openings or holes within respectiveconvolution walls which are in flow aligned to permit the air flow tochange direction and to pass alternately along its flow path in contactwith respective opposite surfaces of the same fin as it moves across andthrough the convolutions. Preferably, the fins are bent or folded inaccordion pleated fashion in terms of shallow, angulated convolutionswith each convolution wall carrying a plurality of holes elongated in adirection at right angles to the flow path of the air stream. Where thefluid carrying tubes are arranged in rows in offset fashion, the airpassage holes within the fins lie intermediate of the tubes for each rowto create air turbulence in line with the tube row.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERREDEMBODIMENT Referring to the drawings, there is illustrated in FIG. 1 avertically oriented heat exchanger 10 such as that forming an evaporatorcoil or a condenser coil for an air conditioning system humidifier, orthe like, in which case, a plurality of elongated refrigerant carryingtubes 12 are connected in serpentine fashion by means of return bendtube portions 14 into a vertically oriented assembly and held in placeby frame members at the top and bottom such as frame member 16 andconstituting a relatively rigid structural array. Only a fragmentaryportion of the heat exchanger is illustrated, the heat exchanger 10being coupled by conventional means to a source of refrigerant whichpasses through the refrigerant carrying tubes in series or parallel flowpaths to effect heat exchange with a secondary external fluid which inthis case is air, passing at right angles to the axis of the tubes 12and to the plane of the paper illustrating the same, FIG. 1. In thisrespect, the frame member 16 maybe provided with annular flanges 18through which the individual tubes pass and in the illustrated portionof the heat exchanger, the return bends l4'may be integral with orjoined to straight tubes 12 approximately at the location of the flaredflanges 18.

The invention is particularly directed to the employment of a pluralityof elongated, thin sheet metal fins 20 which extend longitudinally fromone end of the heat exchanger 10 to the other in close, vertically beformed of aluminum, and. the fins20, is the provision of circular holes22 for each tube, the holes 22 being created by punching material fromthe desired location to permit forming, as illustrated in FIG. 2, anumber of rows of tubes as A, B, C and D to carry the fins, the holes 22of the rows of openings being alternately offset both laterally andlongitudinally in the illustrated embodiment to achieve maximum heattransfer between the air flowing in the direction of arrows 24, FIG. 2,at right angles to the axis of the holes, and the refrigerant or otherconfined fluid within the tubes 12. Further, inorder to effect theconductivity between the fins and the tubes, which are received thereby,in the illustrated embodiment, preferably each tube receiving hole 22,is formed by an annular integral ring 26 formed during stamping,including-a flange portion 28 at bend point 30 with the central tubularportion 32 receiving its tube 12 and forming a low impedance, heatconducting path between the ring 26 and the fin carrying the same. Thesetubular portions and rings are formed by extruding the base sheet metal(fin stock). Intimate contact is then provided between the fin and thetube by expanding the tube about twenty thou sandths inch (0.020 inch).The free edge 36 f the tubular portion 32 of ring 26 is flared outwardlyto permit the fins to readily receive the tubes and be stacked on thetubes during assembly of the heat exchanger.

An important aspect of the present invention resides in the provision ofaccordion pleats or angulated convolutions to the individual fins bybending the .fins along fold lines 38, at right angles to the air flowpath, indicated by arrow 24, to effect shallow angulated convolutiorisin which the adjacent convolution walls 40 extend obliquely in oppositedirections as best seen in FIG. 3. Further, as illustrated in FIG. 3,convolution walls 40 are provided with air passage openings or holes 42intermediate of the fold lines which are generally in flow directionalignment and permit the air to pass from one side of the fin to theother at each convolution for increasing turbulence and heat transferwithstream passing through the heat exchanger 10. As illustrated in FIG.2, the air passage openings or holes 42 within the fins are elongated ina direction at right angles to the air flow path-and are illustrated asbeing flattened in configuration, although of course the size andconfiguration as well as the number of holes for each convolution wallmay readily vary depending upon the design parameters for the particularheat exchanger application. Further, as illustrated in FIG. 2, the airpassage openings or holes 42 form a series intermediate of tube openings22 for a given tube row at all of the four tube location rows A, B, C,or D. As such, air flow impinging against a heat exchange refrigerantcarrying tube 20 passes about the tube on each side thereof and prior toimpacting the tube downstream therefrom and in line therewith, passesthrough the four aligned air passage holes or openings 42 and thricechanges posi-.

tionrelative to the surfaces of the fins to break the boundary layerformed on the surface of the fin and create turbulence in the airpassing through the heat 4 which case the edge of the fin defining theopening 20 in each casedirectly contacts the tube walls to achieve ahigh thermal conductivity joint there'between.

From the above construction, it ,is noted that a substantial portion ofthe fin surface between adjacent tubes 20 carry the punched holes 42 tocause the air flowing through the exchanger to follow a long routerelative to the surface of the fins over which the air passes andthereby increase the heat transfer therebetween. Where the heatexchanger of the present invention is employed in a dehumidifyingfunction, the water removed from the air has a tendency to adhere to thefin surfaces and may accumulate within the punched holes 42. The punchedholes 42 prevent the water blow out and thus the air passingtherethrough may be driven at high face velocities without condensateblow off problems. Where the heat exchanger is employed in anenvironment in which weight is a material factor,since a substantialportion of the fin material is punched out.

to form the holes 42, simultaneously with bending the fins about foldlines 38 in accordion pleated fashion, the net result is the creation ofa heat exchanger whose overall weight is reduced and permitting it tobeused under high heat exchange efficiency in vehicle environments andother environments where weight is an important factor.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

exchanger. This causes the air to diffuse and mix for maximum heattransfer with the refrigerant carried:

What is claimed is:

1. In a finned tube heat exchanger wherein a plurality of fluid carryingtubes extend through a plurality of closely spaced, generally parallelfins formed of metal sheet stock and wherein heat transfer isfacilitated by passing a second exterior fluid across the surfaces ofsaid fins and generally at right angles to the axis of the tubes passingthrough the fins, the improvement wherein:

said fins are convoluted in the direction of flow of said external fluidand carry a plurality of holes therein to permit said second exteriorfluid to pass through the convolutions from side to side for-improvedheat exchange efficiency,

said fins are accordion pleated to define shallow, an-

gulated convolutions, said heat exchange fluid carrying tubes arearranged in multiple, laterally spaced rows with the tubes of adjacentrows being longitudinally offset with respect to each other,

and

said fluid passage holes for respective rows are aligned with the tubeswithin said rows and lie intermediate of the tubes in terms of thedirection of flow of the external fluid.

2. The finned tube heat exchanger as claimed in claim 1, wherein saidheat exchange fluid carrying tubes are arranged in multiple laterallyspaced rows with the tubes of adjacent rows being longitudinally offsetwith respect to each other and wherein said external fluid passage holesfor respective rows are aligned with the tubes within said rows and lieintermediate of the same in terms of the direction of flow of theexternal fluid.

1. In a finned tube heat exchanger wherein a plurality of fluid carryingtubes extend through a plurality of closely spaced, generally parallelfins formed of metal sheet stock and wherein heat transfer isfacilitated by passing a second exterior fluid across the surfaces ofsaid fins and generally at right angles to the axis of the tubes passingthrough the fins, the improvement wherein: said fins are convoluted inthe direction of flow of said external fluid and carry a plurality ofholes therein to permit said second exterior fluid to pass through theconvolutions from side to side for improved heat exchange efficiency,said fins are accordion pleated to define shallow, angulatedconvolutions, said heat exchange fluid carrying tubes are arranged inmultiple, laterally spaced rows with the tubes of adjacent rows beinglongitudinally offset with respect to each other, and said fluid passageholes for respective rows are aligned with the tubes within said rowsand lie intermediate of the tubes in terms of the direction of flow ofthe external fluid.
 2. The finned tube heat exchanger as claimed inclaim 1, wherein said heat exchange fluid carrying tubes are arranged inmultiple laterally spaced rows with the tubes of adjacent rows beinglongitudinally offset with respect to each other and wherein saidexternal fluid passage holes for respective rows are aligned with thetubes within said rows and lie intermediate of the same in terms of thedirection of flow of the external fluid.